1. Field of the Invention
The present invention generally relates to location of obscured conduits, and more particularly to a method of, and apparatus for, determining the location of an underground cable or pipeline which is capable of carrying an alternating electrical current.
2. Description of the Prior Art
It is often necessary to locate buried conduits, such as pipelines for gas, water and sewage, and cables for telephones, in order to repair or replace them if they are damaged or defective. It is also important to know beforehand the location of such underground utilities when excavating. The prior art includes many devices for locating conduits which have electrically conductive cables or are otherwise capable of carrying an electric current; these devices use induction coils or capacitive plates which sense electromagnetic test signals generated by the current in the conductor. The earliest cable locators use a single sensor which detects a single null or peak (depending upon the orientation of the sensor) as the unit passes near the cable. The peak is fairly broad, making it less precise in cable location. While null detection is sharper, it can be misleading where there are multiple cables carrying the test signal, so peak detection is better in such areas but since the response is still very broad, it can be very difficult to find the precise location of the peak.
Many later devices use two or more sensors that combine the signals to provide an indication of conductor proximity. Such sensing systems are described in the following patents:
______________________________________ U. S. Pat. Nos. ______________________________________ 3,617,865 4,542,344 4,091,322 4,639,674 4,134,061 4,665,369 4,220,913 4,672,321 4,295,095 4,686,454 4,387,340 4,843,324 4,390,836 4,942,365 4,427,942 4,990,852 4,438,389 5,001,430 4,458,204 5,093,622 4,520,317 ______________________________________
The last of these patents has a helpful comparison of techniques described in many of the other patents. It will also be appreciated that this latter patent is directed to improvements in signal processing, not in sensor arrangement.
The simplest multi-sensor apparatus is one in which the signals from only two sensors are combined, usually subtractively, to yield a single indication of proximity, as in the first of the above-listed patents. This technique (dual-peak), does not require the complicated circuitry of alternative designs, which is desirable for low-cost, sturdy cable locators. As further illustrated in FIG. 1, a graph of signal strength versus distance from the conductor, the dual-peak method also provides improved sensitivity in the area immediately adjacent to the buried conductor (i.e., a sharper response curve A compared to that B of a single peak detector). This method, however, exhibits false peaks or shoulders on either side of the primary peak, with sharp minima (nulls) between the maxima. This effect is undesirable for several reasons. First, an inexperienced user of the device might make a false reading if the cable is approached from beyond the false peaks. In the exemplary graph of FIG. 1, the cable might be mislocated at a horizontally displaced location two or three times the cable depth. The results might also be falsely read as detecting multiple conductors. Finally, the dual-peak method yields potentially confusing indications in congested areas where more than one conductor may be carrying the test signal.
One prior art device (the '942 patent) masks the side peaks by enabling the speaker only when the lower sensor detects more than a predetermined amount of signal strength than the upper coil detects. Although this method seems appealing, it is not optimum for two reasons. First, the side peaks have been masked, rather than eliminated or transformed into a flat response, which adds a non-linearity to the response and makes the readings particularly confusing and unpredictable in the presence of an adjacent conductor(s) carrying the signal in even small amounts; alternatively, it does not eliminate or reduce the interference from such adjacent cables. Secondly, it again requires additional hardware that increases the cost of the device. It would, therefore, be desirable and advantageous to devise a cable locator which retains the simplicity (and low manufacturing cost) of the dual-peak design, but minimizes the effects of false peaks.